Fast recovery diodes are frequently used in motor control and power supply circuits having an inductive load. It is desirable that such FRDs have a "soft"recovery characteristic; that is, that its di/dt during turn off is limited to reduce the production of high voltage spikes in the inductive load circuit.
FRDs with soft recovery characteristics are described in U.S. Pat. No. 5,747,872 dated May 5, 1998. Those devices employ typical heavy metal doping, for example, gold or platinum, in selective areas of the semiconductor die to reduce lifetime in that area.
Heavy metal doping introduces complex processing steps and is not well controlled. Devices employing heavy metal doping also have a high negative temperature coefficient and are not easily connected in parallel. Further, the use of heavy metal in wafer fabrication facilities can adversely affect the processing operation in other portions of the facility.
It would be desirable to produce an ultra-soft recovery FRD diode in which lifetime in selected regions of the die is limited without the need for heavy metal doping.